Process for coating refractory metals with oxidation-resistant metals

ABSTRACT

A NONADHERENT ELECTROPLATE OF POROUS SPONGY PLATINUM IS ELECTRODEPOSITED UPON A TUNGSTEN SURFACE. THIS MATERIAL IS THEN FUSED IN AN ABNORMAL GLOW-DISCHARGE FURNACE OR TUBE TO PRODUCE A MELTED LAYER OF COATING ON THE SURFACE AND A SOLID SOLUTION BOND BETWEEN THE TWO METALS.

Aug. 6,

Filed Aug. 19. 1969 DISCHARG E VOLTAGE (VOLTS) 1974 c. w. HALDEMAN PROGESS FOR COATING REFRACTORY METALS WITH OXIDATION-RESISTANT METALS 2 Sheets-Sheet 1 NORMAL GLOW l I l I I0 0.0I O.| I I0 IOC DISCHARGE CURRENT (AMPEREs) FIG. I

y INcH DIAMETER HINGED LID 23 SUPPORTING R003 (TUNGSTEN) 2O H gggggm wATER cOoLED vAcUUM 7 CHAMBER UsED As ANoDE 2I GAS 25 I2" WATER COOLED QUARTZ INLET I INSULATOR- I 9m I4 1 --g- -WA'FER COOLEI\D COPPER I I5 CAJ'QHOQE H DER POWER SUPPLY POSITIVE TERMINAL 8 HIGH VOLTAGE 0 RM 22 I CURRENT VACUUM SEALS A; INCH SPACE BETWEEN SZ /l8 7 T INsULAToR AND cATHODE HOLDER SUPPLY POWER SUPPLY NEGATIVE TERMINAL INVENTOR FIG. 2

Aug. 6, 1974 C C.W.HALDEMAN 1 PROCESS FOR COATING REFRACTORY IE'I'ALS' WITH OXIDATIONRESISTANT METALS Filed Aug. 19, 1969 2 sheets -sheet 2 GAS INLET POWER SU PPLY NEGATIVE TERMINAt [/COPPER E NDPLATE v 2 SPRING CLIP QUARTZ INSULATOR '0.03 INCH SPACING 47 L SUPPORTING ROD 4a HIGH VOLTAGE ,L /-QUARTZ JACKET CURRENT LIMITED d C) no SUPPLY 0.003 INCH SPACI NG COPPER ENDPLATE 5831515 UT'EFNYMI NAL VACUUM .44

SYSTEM |NVENTOR= FIG. 3 CHARLES w. HALDEMAN ATTORNEY Mass.

3,827,953 Patented Aug.6, 1974 i'RocEs s F oR COATING REFRACTORY METALS WITH OXIDATION-RESISTAN T META LS Charles W. Haldeman, Lexington, Mass., assigno to Massachusetts Institute of Technology, Cambridge,

Filed Aug. 19, 1969, Ser. No. 851,258

Int. Cl. C23b 5/52 R 7 Claims ABSTRACT on THE DISCLOSURE I I A nonadherent electroplate'of porous spongy platinum is electrodeposited upon a tungsten surface. This material .is then fused in an abnormal glow-discharge furnace or tube to produce a melted layer of coating on the surface and a solid solution bond between the two metals.

BACKGROUND OF INVENTION Field The present invention relates to readily oxidizable metals and improved means for protecting them against oxidation at elevated temperatures.

Description of the Prior Art Refractory metals such as molybdenum, tungsten, tantalum and 'columbium are widely used in aerospace technology because of their extremely high melting points;

however, these metals all suffer from the fact that they are readily oxidized by the composition of air at high temperatures. Oxidized metals are volatile gases that boil off from the surface of the metal above about 1200 F.

rendering the metals virtually useless. Refractory metals also have high thermal electron emission, which makes them unsuitable for use as electrostatic probes for probing plasmas surrounding aerospace vehicles.

' To protect refractory metals from oxidation at high temperatures, oxidation-resistant metals such as platinum are bonded to the refractory metals. While commercial electroplatings of metals such as platinum or iridium can be made on tungsten or other refractory metals, the solutions require a very high level of control and composition and temperature during use. The resulting platings suffer from a lack of density and a lack of adherence to the substrate because the bond is only mechanical. As a result such coatings suffer from cracking and roughening when thicknesses of several mils are built up.

Coatings which are applied by sputtering are somewhat more adherent but again result in merely a mechanical covering of the substrate with the coating material.

Metallurgical bonds may be made by employing shot peening techniques and firing in hydrogen so as to partially diffuse the coating into the metal to be protected.

techniques is a very time consuming process.

SUMMARY It is an object of the invention to produce a strong metallurgical bond between a refractory metal and an oxidation- However, promotion of solid diffusion in the case of these.

surface and the layer, the bond being promoted by the ionic sputtering action bombarding the, surface, the lonic sputtering action maintaining the surface free from oxidation and keeping it-well cleaned and scrubbed.

'Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 represents the voltage-current characteristic of an electric discharge at low pressure.

FIG. 2 is a front elevational view of a glow-discharge furnace suitable for the practice of the invention.

FIG. 3 is a front elevational view of a glow-discharge tube suitable for the practice of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a process which supplies a coating of oxidation-resistant metal, such as platinum, that is extremely adherent, dense, and resistant to oxidation and which can be wetted and bonded well by commercial brazing alloys to refractory metals, such as tungsten. The process includes electroplating platinum upon a tungsten surface and fusing the coating in a hydrogen or helium atmosphere at a pressure between .05 to 5 Torr and at a temperature between 1800" C. and 1900 C. for a few seconds, thereby producing a solid solution bond between the platinum and the tungsten. The electroplating need be neither dense, adherent, nor shiny as long as it remains on the surface well enough to permit transfer to the device where fusion takes place. Thus, delicate and slow plating baths can be eliminated in favor of simpler baths that yield poor deposits. While a chemical cleaning process such as electropolishing, can be employed prior to plating, it is not mandatory as in the case of commercial electroplates; however, as with other coating processes, it may be necessary to use an electropolish or a high mechanical polish in order to achieve the degree of smoothness desired in the end product. Since the final plated surface will be little better in appearance than the original surface, it must be polished if a high polished surface is desired. This polish is not required in the present invention in order to maintain adhesion of the plated surface because the adhesion is achieved by the solid solution bond which is enhanced by the cleaning action of a fusion process.

The fusion process comprises the use of a glow discharge tube or furnace to heat the work. A glow discharge for heating differs markedly from sputtering apparatus in which powers are very low and in which there is no gross heating problem associated with either the workpiece or the chamber in which the work is enclosed. A glow discharge for heating preserves the advantages of complete environmental control and direct heating of the workpiece characteristic of the vacuum induction furnace, without the difficulties inherent in generation of RF power. In contrast to the arc furnace, the glow discharge provides even heating over a wide area 'and does not produce the intense local heating of the arc. Operation of the glow discharge tube or furnace depends upon the fact that in the abnormal glow region of an electric discharge the power input to the cathode surface is very large. The region of the voltage-current characteristic for an electric discharge at pressures well below one atmosphere is shown in FIG. 1. Values are only approximate sincethe exact levels depend on gas and electrode material, pressure, and surface condition. The significant feature of this characteristic is that at low and high currents only one mode of operation is possible. However, at intermediate currents the abnorihai' glow an dai'c both exist, with the arc mode ray proper choice of atmosphere gas,-pressure,'current revel; =and'o'perating procedure, a'discharge in' the abnorr'r'ialglow region can be obtained and used for heating conducting materials to high temperatures. However; in the abnormal glow region of the glow discharge, a very severe insulation problem is imposed upon the area around the cathode because electric fields near the cathode are very high when operating in this mode. Therefore, the discharge will attempt to break through any insulation near the cathode. Accordingly, the insulator must be made of -a high dielectric strength ceramic material, and be spaced within the tube or furnace so as to prevent arcing and transition to the arc mode.

The heating in the glow discharge provides a preferred method of fusing the coating because the sputtering action of the hydrogen ions bombarding the work surface, J

which is the cathode, keeps the surface clean and chemically active. Thus, mixing of the coating metal and the substrate metal is promoted. This action provides the desired solid solution bond in the case of metals exhibiting solid and liquid solubility.

Referring to FIG. 2, the glow discharge furnace comprises a water-cooled vacuum chamber 11 used as the anode. Chamber 11 is provided with an outlet 13 for connection to a vacuum pump (not shown) and an inlet 14 for the introduction of a nonreactive or a reducing gas into the chamber. Shown centrally disposed within the chamber is a water-cooled cathode holder 15 which is electrically insulated therefrom by insulator 21. Insulator 21 is sealed to chamber 11 and cathode holder 15 by ceramic-metal or elastomer O-ring seals 12 and 22 respectively. The cathode holder and insulator are water-cooled in order that the heat absorbed from the glow discharge and heat radiated from the workpiece is dissipated in the walls. Both the insulator and the cathode holder conveniently extend through the bottom of the chamber. The cathode holder must be free from all foreign material to prevent arcing to any discontinuities on its surface. Cathode 16 is comprised of the workpiece to be coated and is made the negative terminal 17 of high voltage current limited D.C. supply 18, the positive pole 19 of which is connected to chamber 11 and to ground. The negative D.C. electrode of the supply is connected to cathode holder 15 at a point where it extends through the bottom of the chamber. Cathode 16 is set on support rod 20,

arc mode. The top of the chamber consists of a tightly fittedhinged lid 23 to permit insertion and removal of the work into and from-the furnace. To prevent arcing along the insulator, the distance from the work 16 to the furnace wall 24, measured along thesinsulator, must be considerably longer than the distance to walls 25, 26, and 27. The work should be; approximately evenly spaced from walls 25, 26, and 27 to maintain as uniform an electric field as possible between the work 16, which is the cathode, and the chamber 11, which is the anode. The preferred spacing between the work and the insulator is /2 inch; increasing this distance willcause a support rod of 2b inch in diameter to be'heated, decreasing this distance will produce an arc insteadof an abnormal glow. The preferred spacing between the work and the cathode holder is one inch; increasing this distance will cause heat to be wasted in heating the-support rod, decreasing this distance will result in excessive heat conduction from the work. A support rod of larger diameter would necessitate "agfeater gw betweeirmearmrk am insula-tormrrctbetween the work and the cathode holder. The location of seal 22 in contact with tin? cathode holder and the insuas the cathode and a water-cooled copper .end plate 42 used as the anode. The end plates are secured to one anotherby means of quartz jacket 43 to form an elongat ed enclosure suitable for heating wires and the like. End plate 42 is provided with outlet 44 for'connection tqa vacuum pump (not shown). End plate 41 is'provided with inlet 45 for the introduction of a nonreactive-gas i'rito'the' enclosure. A tungsten wire or rod 'siipport 47 which is solidly bonded to end plate14'1 suppdr'ts'the workpiece or'wire 48 which comprises part of the cathode. Showri centrally disposed within the enclosure is "insulator" 46 which covers the wire support. A spring clip 49 friction-fitted between the wire support and theinsulator retains the insulator to the wire support. The preferred spacing between the insulator and the workpiece or wire is approximately 0.003 to 0.005 inches to allow for thermal expansion preventing cracking of theinsulatpribyjthe y'vo ferrcd spacing between the insulator'arid' the w pp v is 0.030 inches to provide a ready path-for the escape or outgas. The distance from the workpiece to the anode tions to maintain as uniform an elect, between the cathode andgth e plasma column charge is established. End platefll is'j' made terminal 17 of high voltage. currentlimited, 18, the positive pole 19 ofwhich is connectedtoie '42andtoground.

In operation, insulator 46 prevents current. flowing to wire support 47. Because of the proximity of vthe wire, approximately 2 to 3 inches, to the anode plate,.and. the small diameter of thewire, the principal current flow is to the wire and not the cathode end plate..gThus, the wire and not the cathode end plate is heated'dncontrast to,the furnace, in which the electric field is controlled. by the wall positions, the field in theglass tube, which is .,non conducting to electricity, is controlled, by the plasma column within the tube. V

The workpiece to becoated is prepared by any suitable mechanical cleaning .s uch as grinding; sandblasting, or polishing with an abrasive. If asrnoothsurface desired on the finished coating, electrolytic polishing may;.be. pe r formed in 6 percent potassium hydroxide2.5-; percent sodium nitrate solution ata currentdensity -of :10-amps.to

iidpiaie 30 amps per square centimeter. A. coating of.oxidation-resistant metal. is next electroplated on the cleaned surface by using a suitable bath. The eleetropla-te need heneither dense, adherent nor shiny as long as it remains on the surface to permit transfer to the glow-discharge furnace or tube. After the work is placed on supporting rod 20 or wire support 47'the furnace or tube is evacuated itowat least 10* Torr and purged with hydrogen to removeair. Hydrogen is then introduced to a pressure of about :05 Torr to provide a suitable atmosphere for 1a glow-discharge. The v currentiimited power supply-is energized starting at alow current setting. The current is gradually increased until a bright glow discharge is obtained and the temperature of the workpiece, as observed by an optical pyrometer, increases to about 800 C. When the voltage and current increasessimultaneously, operationis in the abnormal glow mode. Current and pressure are then gradually increased by turning up the power supply and bleeding in hydrogen or helium until the coating is observed to melt. Helium is preferred because it produces a higher voltage discharge than hydrogen. Care must be exercised at this stage to maintain gradual increase in current and temperature of the wor k since emission of electrons from the hot cathode surface appears to be important in maintaining the abnormal glow mode of operation. Should transition to the arc mode occur, the current level and, if necessary, the pressure, must be lowered to reestablish the abnormal glow. A gradual increase in power must be repeated until the desired temperature is reached.

To obtain. a smooth shiny surface on the platinum coating, the power should be shut off immediately after melting the layer of metal. Hydrogen or helium is then bled into the furnace to increase the cooling rate. The work is then permitted to cool down.

Where a high degree of smoothness is required, the process can be repeated starting with a very light strike plating of the coating, a fusion of the coating, and then successively fusing heavier coatings until the desired thickness and surface shine are built up. In successive heatings it is desirable not to have the temperature as high on each successive plating as on the previous plating; that is, the first strike should be heated approximately 300 C. above the melting point, the second strike 200 C. above the melting point, and third 100 C., etc. The raising of the coating to a temperature above the melting point by a significant amount, in case of metals exhibiting solid and liquid solubility, for example, platinum and tungsten, causes a sufficient amount of tungsten to dissolve in the platinum so that it approaches the equilibrium solution concentration. Accordingly, the higher the temperature above the melting point, the more tungsten will penetrate into the platinum or vice-versa. Consequently, by controlling the temperature of melt of successive coatings, it 18 possible to control the rate of transition between the tungsten and platinum. For example, very little tungsten is dissolved into the platinum if the temperature is raised just to the melting point of platinum. Alternatively, a great deal of tungsten is dissolved if the temperature raised signficantly above the melting point. The undercoating, which contains tungsten, will not melt if the temperature is raised to a value below the value for formation of the previous layer because the dissolved tungsten raises the melting point of the platinum-tungsten layer. Thus, it is possible to build up a thicker, smoother plating than if the process were performed in only one heating step in which the liquid layer, being very thick, forms into beads and runs off the surface; whereas, if many thin coatings are applied, the layers do not break up into beads and run off the wet surface.

The minimum voltage necessary to melt the coating into the substrate is dependent upon the particular materials employed as the coating and the substrate. For example, a DC. potential of about 3,000 volts to about 4,000 volts at 6 amperes is required to melt platinum on a tungsten cylinder 0.6 inches in diameter and one inch long. Larger pieces require correspondingly more current. The current-limited power supply used to heat the work preferably has an open circuit voltage of about 5000 volts and an adjustable short circuit current from 1 to amperes.

The pressures required to operate in the abnormal glOW mode are below atmospheric, preferably between .05 and 5 Torr.

All types of refractory high melting metals are suitable for use as the substrate. Examples of high melting metals are molybdenum, tungsten, tantalum, and columbium. Oxidation-resistant materials such as platinum, rhodium,

metals.

The cathode insulator is preferably made of fused quartz and sealed to the chamber and to the cathode holder with O-ring seals. However, other high temperature ceramic materials could be used in place of the fused quartz as could ceramic-tometal seals be used in place of O-rings. The essential nature of the seals is that they be leak free and able to withstand the high heating rates to which they are exposed.

The cathode holder is preferably made of copper. The supporting rod is preferably made of tungsten; the anode of copper or iron. 1

Fusion of an oxidation-resistant metal to a refractory metal could also be achieved by using two separate sources of power; one to heat, the other to clean and scrub the workpiece. As, for example, by applying a lower power glow discharge to scrub and clean a workpiece in a conventional heating furnace in order to enhance the bond.

Several examples of the present invention are described in detail below. These examples are included'to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and scope of the invention. It will be appreciated by those skilled in the art that the main impact of the present invention lies in the use of the abnormal glow mode to heat and clean a workpiece to effect a solid solution bond between those metals exhibiting solid and liquid solubility.

EXAMPLE I A glow discharge furnace, similar to that shown in FIG. 2, except that the chamber and cathode holder were not cooled, was employed wherein a tungsten slug for arc jet nozzles 0.625 inches in diameter and one inch in length was used as the cathode substrate. The dimensions of the furnace used were quite larger than that shown in FIG. 2 which made it possible for intermittent operationwithout water-cooling. The anode consisted of a cylindrical steel vacuum chamber with doors on each end. The cylinder had a diameter of 36 inches and a length of 24 inches. A water-cooled quartz insulator was spaced six inches from the side walls and extended 12 inches through the feed-through wall. The insulator also was spaced from the cathode holder by about /3 inch. A tungsten supporting rod inch in diameter separated the insulator from the work by inch and the cathode holder from the work by one inch. A copper cathode holder supportled the supporting rod which in turn supported the wor A coating of black platinum approximately 0.0001 inch to 0.001 inch thick was electroplated on the cathode substrate using an aqueous solution of chloroplatinic" acid and hydrochloric acid. The electroplate was then transferred to the furnace.

The furnace was evacuated to 10* Torr and purged with hydrogen. Hydrogen was then introduced to a pressure of 0.05 Torr. The current was gradually increased until a bright glow discharge was obtained at approximately 0.1 amperes per square inch and the temperature of the workpiece was observed to be approximately 800 C.

Fusion of the platinum to the tungsten was accomplished by gradually raising the current -to 6 amperes while also increasing the pressure by introducing additional hydrogen. The voltage was approximately;3 000 volts at the point of maximum heating rate. The heating was stopped after the workpiece-reached a temperature of 1800 C. to 1900" C. for a few seconds in theihydrogen atmosphere, thereby melting ,thella'yerlofimetal. The power was then shut off and'hydrogenwasbled into the furnace to permit the work to cool down.

The resultant fused coating was a continuoii's'solid solution bond between the platinum and the tiing's ten"in which some of the tungsten had dissolvedinto the platinum.

7 EXAMPLE II The platinum-coated tungsten slug of Example I was cast in pure oxygen free copper using a graphite mold 2 /2 inches in diameter and about 3 inches high. The apparatus of Example I was employed to fuse the copper to the platinum-coated tungsten. The procedure of Example I was repeated with the following exceptions. IA helium atmosphere was used to prevent solution of the hydrogen in the molten copper and to provide a higher heating rate. The voltage was reduced to 2000 volts at about 6 amperes because of the large size of the graphite mold. The heating was stopped after the workpiece reached a temperature of 1150 C. to 1200 C. whereupon the copper was completely melted.

The castings were dense and of high quality with extremely good bonding between copper and the platinumcoated tungsten. The purpose of the plating on these cylindrical slugs was to facilitate a bond between tungsten and cast high purity copper, which does not wet and bond well to tungsten it pure copper is cast upon bare tungsten. However, when cast upon the platinum-plated tungsten, a solid solution bond is maintained between platinum and tungstenand between platinum and copper. The resultant bond is achieved because solid solubility exists between copper and platinum and between platinum and tungsten. Thus, the fused joint which results from this method of casting a nozzle is a continuous solid solution which changes from pure tungsten to pure copper running through platinum.

EXAMPLE III A glow discharge tube 1 /2 inches in diameter and 12 inches long, similar to that shown in FIG. 3, was employed wherein a tungsten wire of approximately ten mils diameter and four inches in length was used as the cath ode substrate. Fusion of the platinum coating to the tungsten was accomplished by raising the current to about A ampere while also increasing the pressure by introducing additional hydrogen. The voltage was approximately 2500 volts at the point of maximum heating rate. The heating was stopped after the tungsten wire reached a temperature of 1800 C. to 1900 C. for a few seconds in the hydrogen atmosphere.

The resultant platinum coated tungsten wire was used as a Langmiur probe in the MIT wind tunnel where it functioned satisfactorily for approximately one hour at temperatures of approximately 2000 F. After this one hour time period, the integrity of the platinum coating was destroyed as a result of diffusion of the platinum back into the tungsten. This solid state diffusion of platinum in tungsten take place very rapidly in elevated temperatures causing the platinum coating to eventually migrate back into the substrate to form a continuous alloy.

Because of the higher melting point of iridium, iridum coatings should hold up for longer periods of time than platinum coatings when exposed to high temperatures.

EXAMPLE IV The apparatus and procedure of Example 11 was repeated employing a tantalum wire of approximately ten EXAMPLE v A thicker coat of platinum was built up on the tungsten 'wireof Example III to achieve oxidation protection for longer periods and a higher degree of smoothness by repeating the process of Example III starting with a very light strike plating of black platinum. The first strike was heated well above the melting point at 2000 C. to 2500 C. for several minutes. Successive strikes were heated about C. lower than each previous strike for shorter time periods until the desired thickness of 1 mil was built up. By this procedure each under-layer did not melt when subsequent layers were applied. The last layer was applied as specified in Example III.

While the invention has been described in detail in the foregoing specification and the drawing similarly illustrates the same, the aforesaid is by way of illustration only and is not restrictive in character.

What is claimed is:

1. A process for coating a substrate surface of a refractory metal selected from the group consisting of tungsten, tantalum, molybdenum, and columbium which comprises the steps of (a) cleaning the substrate surface,

(b) electrodepositing a layer of platinum-group metal upon the cleaned surface, and

(c) abnormal glow-discharge heating the electrodeposited surface in a protective atmosphere at a pressure between .05 and 5 Torr by raising it to a temperature high enough to cause the layer to melt fora few seconds to produce a melted layer of coating on the substrate surface and a solid solution bond between the surface and the layer, the bond being promoted by bombardment of the surface by highenergy ions, thus maintaining the surface free from oxidation and keeping it well cleaned and scrubbed.

2. A process, as recited in claim 1, wherein the electro deposited layer is a loose coating approximately 0.0001 inch to 0.001 inch thick.

3. A process, as recited in claim 1, wherein the protective atmosphere is hydrogen.

4. A process, as recited in claim 1, wherein the protective atmosphere is helium.

5. A process, as recited in claim 1, wherein the glowdischarge heating includes:

(a) employing a platinum-coated tungsten slug as the cathode in a glow-discharge furnace;

(b) evacuating the furnace to at least 10* Torr;

(c) purging the furnace with hydrogen to remove air;

(d) introducing a nonreactive gas to a pressure of about 0.05 Torr to provide a suitable atmosphere for a glow-discharge to take place;

(e) energizing, starting at a low current setting, a current-limited power supply;

(f) increasing the current gradually until a bright glow discharge is obtained at approximately 0.1 amperes per square inch, the temperature of the cathode being approximately 800 0.;

(g) further increasing the current together with the pressure gradually to heat the cathode to a temperature between 1800 C. and 1900 C. and maintaining that temperature for a few seconds to produce a melted layer of fused coating on the surface;

(h) shutting down the power supply immediately after melting the layer of metal; and

(i) bleeding a nonreactive gas into the furnace to cool the cathode.

6. A process, as recited in claim 5 wherein the nonreactive gas is helium.

7. A method of applying a protective coating to a substrate of a refractory metal, which substrate acts as a cathode surface, that comprises, depositing a layer of an oxidation-resistant metal upon the surface of the substrate, abnormal glow-discharge heating'the deposited surface in a protective atmosphere between a pressure high enough to provide suflicient current density to melt the oxidationresistant metal and low enough to prevent arcing, and establishing a temperature due to said abnormal glowdischarge heating high enough and for a sufiicient time to cause the layer to melt to produce a melted layer of 9 10 coating on the substrate surface thus promoting a solid References Cited solution bond between the surface and the layer, the heat- UNITED STATES PATENTS ing period being short enough to prevent run-off of the 2,555,372 1951 Ramage 204 7 melt, said bond being promoted by ions derived from 5 5255;112:5251 et al. "23kg: the protective atmosphere bombarding the surface, said 2:739:107 3/1956 Ricks R abnormal glow-discharge being effected with a discharge 3 514 383 5 1970 Bmmfield et a1 tdn't of tle tfift nm'll'am e e cunfm 6 S1 y a as ee 1 8 per squar JOHN H. MACK, Primary Examiner centimeter of cathode surface, and increasing the level of W I SOLOMON A E the discharge current density to a value sufiicient to melt Sslstant Xammer the oxidation resistant material and low enough to pre- US. Cl. X.R. vent arcing. 117-93.1 GD; 204-192 

